Exposure apparatus and method of cleaning optical element of the same

ABSTRACT

An exposure apparatus in which exposure process is performed by projecting an exposure beam of X-rays or ultraviolet rays from an excimer laser, for example, to a substrate, is provided with an arrangement wherein an inert gas containing a small amount of oxygen is supplied to a closed space where an optical element is disposed, and wherein the exposure beam is projected thereto by which ozone is produced in the closed space. Organic compound deposited on the optical element can be removed by photochemical reaction through projection of the exposure beam and the produced ozone.

FIELD OF THE INVENTION AND RELATED ART

[0001] This invention relates generally to an exposure apparatus whichuses, as exposure beam, short wavelength electromagnetic waves such asX-rays or ultraviolet rays from an excimer laser, for example. Morespecifically, the invention is concerned with such exposure apparatus, adevice manufacturing method using the same, and a method of clearing anoptical element of such exposure apparatus.

[0002] In projection exposure apparatuses for manufacture ofsemiconductor integrated circuits, light of various wavelength bands isprojected as exposure beam to a substrate. As for such exposure beam,examples are e-line (wavelength λ=546 nm), g-line (λ=436 nm), h-line(λ=405 nm), i-line (λ=365 nm), KrF excimer laser (λ=248 nm), ArF excimerlaser (λ=193 nm), and X rays.

[0003] An exposure beam emitted from a light source is directed by wayof an illumination optical system for illuminating a reticle and aprojection optical system (projection lens) for imaging a fine patternformed on the reticle upon a photosensitive substrate, whereby the finepattern is lithographically transferred to the photosensitive substrate.In such exposure apparatuses, miniaturization of pattern linewidth hasforced further improvements of throughput and resolution. Also, anexposure beam of higher power has been required and, on the other hand,shortening of wavelength band of exposure beam has been required.

[0004] It is known that when an exposure beam of i-line (wavelengthsλ=365 nm) or a shorter wavelength is used, due to shortening of thewavelength, impurities in the air photochemically may react with oxygen.The product (blurring material) of such reaction may be deposited on anoptical element (lens or mirror) of the optical system, causingnontransparent “blur”.

[0005] As regards such blurring material, in a case where sulfurous acidSO₂ absorbs light energy and it is excited thereby, a typical examplemay be ammonium sulfate (NH₄)₂SO₄ produced by reaction (oxidization)with oxygen in the air. When such ammonium sulfate is deposited on thesurface of an optical element such as a lens or mirror, theabove-described “blur” results. Then, the exposure beam is scattered orabsorbed by ammonium sulfate, so that the transmission factor of theoptical system decreases. This causes a large decrease of light quantity(transmission factor) upon the photosensitive substrate, and thus adecrease of throughput.

[0006] Particularly, for ArF excimer laser (193 nm) or X-rays which arein a very short wavelength region, the exposure beam may cause strongphotochemical reaction. Thus, the above-described problems is veryserious.

SUMMARY OF THE INVENTION

[0007] Japanese Laid-Open Patent Application, Laid-Open No. 216000/1994shows an arrangement wherein a barrel having mounted therein a glassmember such as a lens is provided in a casing of closed structure, andwherein the inside of the casing is filled with an inert gas, thereby tosolve the problem such as described above.

[0008] However, it has been found that, in such example using inert gas,an optical element within the barrel or casing of the illuminationoptical system may be contaminated by organic molecules. These moleculesmay be those of any solvent, for example, used during manufacturing andworking processes of components of the illumination optical system andremaining on the components, or those of adhesive agent used in thecasing or barrel and evaporated therefrom.

[0009] Taking the manufacturing procedure into consideration, theenvironmental air may be contaminated by organic molecules coming froman adhesive agent layer between a substrate and a photoresist, forexample. These molecules may enter the casing or barrel. Even if theorganic molecules are at a low concentration, particles may bedecomposed due to the influence of ultraviolet beam and they may bedeposited on the optical element. In that occasion, a carbon film or afilm containing carbon will be produced on the element surface.

[0010] Japanese Laid-Open Patent Application, Laid-Open No. 209569/1995shows an arrangement wherein, when an inert gas is supplied into aprojection optical system, a small amount of ozone is mixed into theinert gas, such that an inert gas containing ozone is supplied to anoptical system. The optical element is irradiated with an exposure beamin a gas ambience containing ozone and, due to ozone cleaning effect,decomposition of organic molecules on the surface of the optical elementas well as deposition of product of decomposition thereon are prevented.

[0011] In this structure, however, an ozone generator having a Hg lampis provided in a portion of an inert gas supplying line. The ozonegenerator produces ozone beforehand, and then the ozone is supplied intothe lens holder. This structure needs use of two light sources, that is,the exposure light source and the ozone generating light source. Thestructure is thus complicated. Further, this creates the followingdangerous possibilities. That is, ozone has a property for deterioratingan element. Therefore, the ozone generator itself may be easily damagedby the influence of ozone. Thus, there is a possibility of leakage ofharmful ozone from the damaged ozone generator.

[0012] It is an object of the present invention to provide an exposureapparatus by which contamination of an optical element by organicmolecules can be prevented, particularly, very simply and effectively.

[0013] It is another object of the present invention to provide a devicemanufacturing method using such exposure apparatus and/or a method ofcleaning an optical element of an exposure apparatus.

[0014] In accordance with an aspect of the present invention, there isprovided an exposure apparatus, comprising: a light source for producingan exposure beam; an optical system having a closed space, forprojecting the exposure beam to a substrate for exposure thereof; firstsupplying means for supplying an inert gas into the closed space of saidoptical system; and second supplying means for supplying one of oxygenand a clean air, so that the inert gas and oxygen can be supplied to theclosed space.

[0015] In accordance with another aspect of the present invention, thereis provided a device manufacturing method comprising the steps of:preparing an exposure apparatus as recited above; and performing anexposure process by use of the exposure apparatus.

[0016] In accordance with a further aspect of the present invention,there is provided a method of cleaning an optical element of an exposureapparatus for exposing a substrate with an exposure beam of ultravioletrays or X-rays, projected thereto, said method comprising the steps of:supplying an inert gas containing a small amount of oxygen into a spacein which the optical element is placed; projecting the exposure beam sothat ozone is produced in the space; and removing an organic compounddeposited on the optical element through photochemical reaction byprojection of the exposure beam and the produced ozone.

[0017] These and other objects, features and advantages of the presentinvention will become more apparent upon a consideration of thefollowing description of the preferred embodiments of the presentinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a schematic view of a general structure of an exposureapparatus according to an embodiment of the present invention.

[0019]FIG. 2 is a sectional view for explaining an example of insidestructure of a barrel.

[0020]FIG. 3 is a flow chart of device manufacturing processes.

[0021]FIG. 4 is a flow chart for explaining details of a wafer process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] Preferred embodiments of the present invention will now bedescribed with reference to the accompanying drawings. In an embodimentof the present invention to be described below, the invention is appliedto an exposure apparatus of reduction projection type, which isgenerally called a stepper or a scanner.

[0023] The exposure apparatus A1 has a major assembly which may beroughly separated into a light source 1 (ArF excimer laser lightsource), a light source lens system 2 which comprises an illuminationoptical system for transforming laser light L1, which is illuminationlight emitted from the light source 1, into light of a predeterminedshape, and a projection lens system 5 for imaging the laser light L1,having been formed into a predetermined shape by the light source lenssystem 2, upon a wafer 4 (substrate) through a reticle 3. The lightsource 1 includes a laser control device 6 for controlling laser outputof the same. The laser control device 6 is controlled by a controller 7(control means). The laser control device 6 functions to change theemission laser wavelength region, to be described later.

[0024] In this embodiment, the light source comprises ArF excimer laserwhich produces ultraviolet rays. However, it may comprise a KrF excimerlaser light source or, alternatively, an X-ray source for producingshorter wavelength X-rays (X-rays are referred generally to soft X-raysor vacuum ultraviolet rays, for example), such as a synchrotronradiation source or a laser plasma radiation source, for example.

[0025] The light source lens system 2 comprises barrels 2 h and 2 i inwhich a plurality of lenses 2 a 2 d are disposed therewithin. Thesebarrels are accommodated in a casing 2 g. Disposed on the opposite sidesof them are mirrors 4 a and 4 b for deflecting and directing the laserlight along a path. The casing 2 g of the illumination optical systemhas a window 2 e demountably mounted at an entrance opposed to the lightsource L and also a window 2 f at an exit opposed to a reticle 3.

[0026] The projection optical system 5 has a plurality of lenses 5 a and5 b accommodated in a lens barrel, for projecting a pattern of thereticle onto a wafer in a reduced scale. There are windows 5 c and 5 dmounted on the faces opposed to the reticle and the wafer, respectively.

[0027] Inert gas of nitrogen gas (it may be a gas of helium or neon, forexample) is supplied to an inside space 2 j of the casing 2 g of theillumination optical system 2, to the inside spaces of the barrels 2 hand 2 i, and to the spaces inside the projection lens 5 and the lensbarrel as separated by lenses. Inert gas supplying device 8 a isconnected to these spaces through an inert gas supplying line 8 b and anelectromagnetic valve 8 c (opening/closing valve) provided in a portionof this line. At an intermediate position on the inert gas supplyingline 8 b, there is an oxygen supplying line 10 b connected as a branch.Through an electromagnetic valve 10 c (opening/closing valve) providedin a portion of this line, an oxygen supplying device 10 a is connectedthereto. This enables mixture of a small amount of oxygen into the inertgas to be supplied. In place of pure oxygen, a clean air which containsoxygen may be mixed.

[0028] There is a gas discharging device 9 a for discharging gases fromthese spaces to which inert gas is supplied. These spaces are connectedto the gas discharging device 9 a through a gas discharging line 9 b andan electromagnetic valve 9 c (opening/closing valve) provided in aportion of this line.

[0029] The gas collected by the gas discharging means may include asmall amount of residual ozone. The gas discharging device 9 a may beprovided with converter means for re-converting the residual ozone intooxygen. Impurities may be removed by a filter and thereafter the oxygenthus re-converted may be circulated to the oxygen supplying device 10 a,for reuse thereof.

[0030] The electromagnetic valves 8 c and 9 c provided in portions ofthe line for the inert gas supplying means and for the gas dischargingmeans are controlled in accordance with a program set in the controller7 beforehand, so that the spaces are kept in a state as filled withinert gas, during normal operation of the apparatus (exposure process)or a stand-by period.

[0031] More specifically, in accordance with preset timing, in thestand-by state of the apparatus, the electromagnetic valve 10 c of theoxygen supplying line opens so that a small amount of oxygen is mixedinto the nitrogen gas. The resultant gas is supplied into the casing andbarrels of the illumination optical system and also into the barrel ofthe projection optical system. The opening/closing of theelectromagnetic valve is controlled so that the amount of oxygen supplyis kept not greater than a predetermined concentration (e.g., notgreater than a a few grams per lm³). After the mixture gas is supplied,the electromagnetic valves 8 c and 9 c are closed. In the state in whichthe spaces are filled with a gas in which a small amount of oxygen ismixed into a nitrogen gas, projection of laser light is performed. Inresponse to this, in these spaces, oxygen within the inert gas, fillingthe space, is converted into ozone through photochemical reaction. Thus,in these spaces, ozone is produced first. Laser projection is continuedin this state and, as a result, any organic compound deposited onoptical elements (lenses, mirrors or windows) constituting the opticalsystem is oxidized. Consequently, organic molecules on the opticalelement is removed by ozone cleaning, whereby the optical element iscleaned.

[0032] Subsequently, the electromagnetic valves on the inert gassupplying line and the gas discharging line are opened, and supply ofinert gas and discharging of the gas are continued interruptedly oruninterruptedly until the inside gas is completely replaced by nitrogengas. The series of these sequential operations are performed inaccordance with a program set in the controller beforehand. As regardsthe cleaning of optical elements, it may preferably be made during thestand-by period of the apparatus in which the apparatus is heldinoperative, since it does not influence the throughput. Alternatively,cleaning may be made during actual operation of the apparatus.

[0033] The efficiency of producing ozone from oxygen by light projectionlargely depends on the wavelength of light projected. In considerationof this, in order to assure efficient production of ozone, thewavelength of exposure beam may preferably be changed, between theexposure process for a substrate and the cleaning operation for opticalelements. More specifically, for the cleaning operation, preferably thewavelength region may be oscillated continuously or it may be changed toshorter wavelength side, by which the ozone production efficiency andthus the cleaning capacity can be improved. Changing the wavelength maybe accomplished by controlling the light source actuation or byinserting wavelength changing means (such as a harmonic wave producingelement, for example) into the light path, for example.

[0034]FIG. 2 is an enlarged view of a portion of the barrel 2 h of theillumination optical system, in the neighbourhood of a gas blowing portof the inert gas supplying line. As regards optical elements 13 a and 13b, it is expected that a large amount of organic molecules may bedeposited on these elements. Thus, the structure is so arranged that theinert gas is directly blown against these optical elements. Thisenhances the ozone cleaning effect considerably.

[0035] The inert gas supplying line may be provided with flow ratechanging means such as disclosed in Japanese Laid-Open PatentApplication, Laid-Open No. 216000/1994, so that nitrogen can be suppliedat a large flow rate for the impurity removing operation.

[0036] Exposure apparatus may include optical elements other thanlenses, mirrors or windows as described. An example is a filter fortransmitting only a desired wavelength of light among a broad wavelengthband as emitted by a light source such as Hg lamp or synchrotronradiation source. The advantageous effects of the present inventiondescribed above similarly applies to such optical element.

[0037] Next, an embodiment of a device manufacturing method which usesan exposure apparatus such as described above, will be explained.

[0038]FIG. 9 is a flow chart of procedure for manufacture ofmicrodevices such as semiconductor chips (e.g. ICs or LSIs), liquidcrystal panels, CCDs, thin film magnetic heads or micro-machines, forexample.

[0039] Step 1 is a design process for designing a circuit of asemiconductor device. Step 2 is a process for making a mask on the basisof the circuit pattern design. Step 3 is a process for preparing a waferby using a material such as silicon. Step 4 is a wafer process which iscalled a pre-process wherein, by using the so prepared mask and wafer,circuits are practically formed on the wafer through lithography. Step 5subsequent to this is an assembling step which is called a post-processwherein the wafer having been processed by step 4 is formed intosemiconductor chips. This step includes assembling (dicing and bonding)process and packaging (chip sealing) process. Step 6 is an inspectionstep wherein operation check, durability check and so on for thesemiconductor devices provided by step 5, are carried out. With theseprocesses, semiconductor devices are completed and they are shipped(step 7).

[0040]FIG. 10 is a flow chart showing details of the wafer process.

[0041] Step 11 is an oxidation process for oxidizing the surface of awafer. Step 12 is a CVD process for forming an insulating film on thewafer surface. Step 13 is an electrode forming process for formingelectrodes upon the wafer by vapor deposition. Step 14 is an ionimplanting process for implanting ions to the wafer. Step 15 is a resistprocess for applying a resist (photosensitive material) to the wafer.Step 16 is an exposure process for printing, by exposure, the circuitpattern of the mask on the wafer through the exposure apparatusdescribed above. Step 17 is a developing process for developing theexposed wafer. Step 18 is an etching process for removing portions otherthan the developed resist image. Step 19 is a resist separation processfor separating the resist material remaining on the wafer after beingsubjected to the etching process. By repeating these processes, circuitpatterns are superposedly formed on the wafer.

[0042] With these processes, high density microdevices can bemanufactured.

[0043] In accordance with an exposure apparatus of the presentinvention, the problem of deposition of organic molecules on an opticalelement and reduction of illuminance thereby can be removed. Since theexposure beam itself to be used for the exposure process is used alsofor ozone production, use of additional and large mechanism is notnecessary. Further, since ozone production is made only inside a closedspace in which the optical element is disposed, there is no dangerouspossibility of leakage of harmful ozone.

[0044] Since the exposure apparatus provides high throughput constantly,a high productivity is accomplished in the device manufacture.

[0045] While the invention has been described with reference to thestructures disclosed herein, it is not confined to the details set forthand this application is intended to cover such modifications or changesas may come within the purposes of the improvements or the scope of thefollowing claims.

What is claimed is:
 1. An exposure apparatus, comprising: a light sourcefor producing an exposure beam; an optical system having a closed space,for projecting the exposure beam to a substrate for exposure thereof;first supplying means for supplying an inert gas into the closed spaceof said optical system; and second supplying means for supplying one ofoxygen and a clean air, so that the inert gas and oxygen can be suppliedto the closed space.
 2. An apparatus according to claim 1 , wherein saidoptical system comprises one of an illumination optical system and aprojection optical system.
 3. An apparatus according to claim 1 ,wherein said light source comprises one of a light source for producingultraviolet rays and a light source for producing X-rays.
 4. Anapparatus according to claim 3 , wherein said light source comprises anArF excimer laser light source.
 5. An apparatus according to claim 1 ,wherein the inert gas comprises one of a nitrogen gas, a helium gas, anda neon gas.
 6. An apparatus according to claim 1 , further comprisingmeans for mixing one of oxygen and a clean air into the inert gas.
 7. Anapparatus according to claim 6 , further comprising a supplying line forsupplying the inert gas into the closed space, wherein the oxygen orclean air is introduced from a line branched from the supplying line,for mixture of the same into the inert gas.
 8. An apparatus according toclaim 1 , further comprising means for discharging the gas inside theclosed space outwardly.
 9. An apparatus according to claim 8 , furthercomprising means for transforming ozone remaining in the discharged gasinto oxygen for reuse thereof.
 10. An apparatus according to claim 1 ,further comprising means for changing the wavelength of the exposurebeam.
 11. An apparatus according to claim 10 , wherein, when the oxygenis supplied, the wavelength of exposure beam is changed into awavelength region higher than an oxygen absorptivity.
 12. An apparatusaccording to claim 10 , wherein, when the oxygen is supplied, thewavelength of exposure beam is changed to s shorter wavelength.
 13. Adevice manufacturing method comprising the steps of: preparing anexposure apparatus as recited in claim 1 ; and performing an exposureprocess by use of the exposure apparatus.
 14. A method according toclaim 13 , further comprising applying a resist to a substrate, beforethe exposure process, and developing the substrate after the exposureprocess.
 15. A method of cleaning an optical element of an exposureapparatus for exposing a substrate with an exposure beam of ultravioletrays or X-rays, projected thereto, said method comprising the steps of:supplying an inert gas containing a small amount of oxygen into a spacein which the optical element is placed; projecting the exposure beam sothat ozone is produced in the space; and removing an organic compounddeposited on the optical element through photochemical reaction byprojection of the exposure beam and the produced ozone.
 16. A methodaccording to claim 15 , further comprising changing a wavelength of theexposure beam, between exposure of the substrate and the cleaning of theoptical element.
 17. A method according to claim 16 , wherein thewavelength of exposure beam is changed by one of controlling actuationof a light source for producing the exposure beam and insertingwavelength changing means to a light path.
 18. A method according toclaim 15 , further comprising discharging the removed organic compoundoutwardly of the space.